October 2017 - Double Star of the Month

William Sadler Franks was an astronomer who assisted Isaac Roberts at his observatory in Crowborough from 1892 to 1906 and then went on the act as astronomer for Mr. F. J. Hanbury in East Grinstead from 1908.

One of his interests was in the micrometrical measurement of wide double stars and he undertook to re-observe all the pairs in the Dorpat and Pulkova appendix catalogues which are usually marked by the characters OΣΣ, Σ I and Σ II.

The results, which were achieved with a Cooke telescope a little over 6-inches in diameter and a filar micrometer, appear in eight lists published in Monthly Notices of the RAS between 1914 and 1920.

During the course of this work he swept up a number of new, wide systems. The WDS catalogue contains the pairs FRK 1 to FRK 13 but in fact there are only 10 pairs in this list now left in the catalogue.

FRK 11 (22 30 06.50 +49 21 23.1) lies in Lacerta just over 1 degree south and slightly preceding α Lac. The primary is V = 6.7 and it is a K2 giant star. The companion, which is optical but also a K giant, is currently 67 arc-seconds distance in PA 91 degrees and is considerably fainter - magnitude 10.7. The interest attached to this pair is that the brighter star is an RS CVn binary with a period of 17.76 days and an amplitude of 0.12 in V.

In the south-eastern corner of Grus are a number of double stars discovered by James Dunlop at Parramatta.

Three degrees east of ε Gruis is DUN 246, a fine pair of yellow stars of magnitudes 6.3 and 7.1 currently 9 arc-seconds apart in PA 254 degrees.

Move a further two degrees east and you will alight on DUN 248 (23 20 50.13 -50 18 23.0). The stars in this case are magnitude 6.2 and 6.6 and measured by me at 211.4 degrees and 16".90 in late 2013, but in 1947, Richard Rossiter discovered that the A star was a close pair.

At that time the separation was 0".5 but in the 70 years since the separation has widened and the stars are now 1".3 apart. The companion is magnitude 8.9 so it will need probably 25-cm to see this close pair.

Bob Argyle - Double Star Section Director